U.S. patent number 4,785,441 [Application Number 07/004,139] was granted by the patent office on 1988-11-15 for tracking error correction apparatus utilizing phase difference information derived from four photocells.
This patent grant is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Haruo Isaka, Toshiki Itoi, Shinichi Tanaka, Hiroshi Yasuda.
United States Patent |
4,785,441 |
Tanaka , et al. |
November 15, 1988 |
Tracking error correction apparatus utilizing phase difference
information derived from four photocells
Abstract
Disclosed herein is an optical information reproducing apparatus
having a phase difference tracking system which is free from an
influence of a tangential phase difference of output signals from a
photodetector. The photodetector comprises first through fourth
photocells which are respectively disposed at first through fourth
quadrants defined by a first straight line parallel to an
information track image and a second straight line perpendicular to
the first straight line. Output signals of the first and second
photocells are delayed for a specified time by respective first and
second delaying circuits. Output signals of the first delaying
circuit and the third photocell are added by a first adder, and
output signals of the second delaying circuit and the fourth
photocell are added by a second adder. A phase comparator produces
a tracking error signal from a phase difference between output
signals of the first and second adders. Alternatively, a first
phase comparator produces a phase difference between the output
signals of the first and second photocells, and/or a second
comparator produces a phase difference between the output signals
of the third and fourth photocells. At least one of output signals
of the first and second comparator is used for producing a tracking
error signal.
Inventors: |
Tanaka; Shinichi (Kyoto,
JP), Isaka; Haruo (Hirakata, JP), Yasuda;
Hiroshi (Hirakata, JP), Itoi; Toshiki (Shiki,
JP) |
Assignee: |
Matsushita Electric Industrial Co.,
Ltd. (Osaka, JP)
|
Family
ID: |
11645868 |
Appl.
No.: |
07/004,139 |
Filed: |
January 16, 1987 |
Foreign Application Priority Data
|
|
|
|
|
Jan 16, 1986 [JP] |
|
|
61-6711 |
|
Current U.S.
Class: |
369/44.23;
G9B/7.089; G9B/7.066; 369/44.41 |
Current CPC
Class: |
G11B
7/0901 (20130101); G11B 7/094 (20130101); G11B
7/0909 (20130101) |
Current International
Class: |
G11B
7/09 (20060101); G11B 007/00 () |
Field of
Search: |
;369/43-47 ;250/21DF
;358/342 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
4375088 |
February 1983 |
de Haan et al. |
4541082 |
September 1985 |
Horkoshi et al. |
|
Primary Examiner: Richardson; Robert L.
Attorney, Agent or Firm: Wenderoth, Lind & Ponack
Claims
What is claimed is:
1. An optical information reproducing apparatus for optically
reading an information recorded as a succession of pits formed on
an information track on a recording medium, comprising:
means for forming a light spot on said recording means;
converging means for collecting a reflected light from said light
spot to form a far field image of said light spot;
a photodetector for receiving said far field image, said
photodetector comprising first through fourth photocells
respectively disposed at first through fourth quadrants defined by
a first straight line parallel to a direction in which said
information track is projected and a second straight line
perpendicular to said first straight line, where said first
straight line defines a border between said first and fourth
quadrants and between said second and third quadrants, and said
second straight line defines a border between said first and second
quadrants and between said third and fourth quadrants;
means for producing an information signal corresponding to said
information from an output signal of said photodetector;
tracking error detecting means comprising: first delaying means for
delaying an output signal of said first photocell for a specified
time; second delaying means for delaying an output signal of said
second photocell for said specified time; first adding means for
adding an output signal of said first delaying means and an output
signal of said third photocell; second adding means for adding an
output signal of said second delaying means and an output signal of
said fourth photocell; and phase comparison means for producing, as
a tracking error signal, a phase difference signal corresponding to
a difference of phases of output signals of said first and second
adding means; and
means responsive to said tracking error signal for driving said
light spot forming means to correct a tracking error.
2. The optical information reproducing apparatus as claimed in
claim 1, wherein each of said first and second delaying means
comprises a primary low-pass filter.
3. The optical information reproducing apparatus as claimed in
claim 1, wherein each of said pits has a phase depth less than a
quarter wave length, and wherein said specified time by said
delaying means is set larger than a phase difference between an
output signal from a photocell arranged in one side divided by said
second straight line and an output signal from a photocell arranged
in the other side when said light spot is correctly focused.
4. The optical information reproducing apparatus as claimed in
claim 1, further comprising: optical means provided in said light
converging means for introducing astigmatism in said far field
image; focussing error detecting means for producing a focussing
error signal from a level difference between low frequency
components of the output signals of said first and second adding
means; and means responsive to said focussing error signal for
driving said light spot forming means to correct a focussing
error.
5. The optical information reproducing apparatus as claimed in
claim 1,, wherein each of said first and second deplaying means is
a variable delaying means, and wherein said apparatus further
comprising delaying time control means for controlling a delaying
time of said variable delaying means according to a phase
difference between a sum of the output signals of said first and
second delaying means and a sum of the output signals of said third
and fourth photocells.
6. An optical information reproducing apparatus for optically
reading an information recorded as a succession of pits formed on
an information track on a recording medium, comprising:
means of forming a light spot on said recording means;
converging means for collecting a reflected light from said light
spot to form a far field image of said light spot;
a photodetector for receiving said far field image, said
photodetector comprising first through fourth photocells
respectively disposed at first through fourth quadrants defined by
a first straight line parallel to a direction in which said
information track is projected and a second straight line
perpendicular to said first straight line, where said first
straight line defines a border between said first and fourth
quadrants and between said second and third quadrants, and said
second straight line defines a border between said first and second
quadrants and between said third and fourth quadrants;
means for producing an information signal corresponding to said
information from an output signal of said photodetector;
tracking error detecting means for producing a tracking error
signal from at least one of a phase difference between output
signals of said first and second photocells and a phase difference
between output signals of said third and fourth photocells; and
means responsive to said tracking error signal for driving said
light spot forming means to correct a tracking error.
7. The optical information reproducing apparatus as claimed in
claim 6, wherein said tracking error detecting means comprises: a
first low-pass filter for passing a low frequency component of an
output signal of said third photocell; a second low-pass filter for
passing a low frequency component of an output signal of said
fourth photocell; first adding means for adding an output signal of
said first photocell and an output signal of said first low-pass
filter; second adding means for adding an output signal of said
second photocell and an output signal of said second lowpass
filter; and phase comparison means for producing, as said tracking
error signal, a phase difference signal between output signals of
said first and second adding means.
8. The optical information reproducing apparatus as claimed in
claim 7, further comprising: optical means provided in said
converging means for introducing astigmatism into said far field
image; focussing error detecting means for producing a focussing
error signal from a level difference between low frequency
components of the output signals of said first and second adding
means; and means responsive to said focussing error signal for
driving said light spot forming means to correct a focussing
error.
9. The optical information reproducing apparatus as claimed in
claim 6, wherein said tracking error detecting means comprises:
first phase comparison means for producing a first phase difference
signal corresponding to a phase difference between output signals
of said first and second photocells; second phase comparison means
for producing a second phase difference signal corresponding to a
phase difference between output signals of said third and fourth
photocells; and adding means for adding said first and second phase
difference signals thereby to obtain said tracking error
signal.
10. The optical information reproducing apparatus as claimed in
claim 9, further comprising: optical means provided in said
converging means for introducing astigmatism into said far field
image; focussing error detecting means for producing a focussing
error signal from a level difference between a sum of the output
signals of said first and third photocells and a sum of the ouput
signals of said second and fourth photocells; and means responsive
to said focussing error signal for driving said light spot forming
means to correct a focussing error.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical information reproducing
apparatus which optically reads an information from a recording
medium, and more particularly to its tracking control.
2. Description of the Prior Art
There is a well known method to carry out tracking control required
for an optical information reproducing apparatus, in which
two-divided parts of a photodetector differentially detects, an
unbalanced quantity distribution of reflected light from an
information track to fetch signals corresponding to a tracking
error. Such a tracking error detecting method is disclosed, for
example, in U.S. Pat. No. 4,059,841. The method, however, has a
problem that the tracking control tends to be unstable due to a
variation of offset of the tracking error signals caused by an
inclination of the recording medium or a displacement of an image
on the photodetector.
There is also a well known method which divides a photodetector
into four quadrants and detects a tracking error by a phase
difference between a signal corresponding to a sum of quantities of
lights incident on the first and third quadrants and a signal
corresponding to a sum of quantities of lights incident on the
second and fourth quadrants. (This method will be hereinafter
called "phase difference tracking method".) This phase difference
tracking method is disclosed, for example, in Japanese Pat. Appln.
of laid-open No. 52-93222. The method can substantially solve the
problem previously mentioned. However, it has still a problem that,
when reading an information from a recording medium in which the
information is recorded as irregularities on a surface with a phase
depth less than a quarter wave length, a displacement of an image
on the photodetector causes a variation of offset of the tracking
error signal, resulting in a tendency to unstable tracking control.
We inventors discovered that the problem was due to the following
reason.
When the far field image of a light spot formed on a photodetector
is divided into four quadrants by a first straight line
substantially parallel to a direction in which an information track
is extended and a second straight line substantially perpendicular
to the direction, there is a phase difference between quantity
changes of light incident on the first and second quadrants located
on one side to the second line and of light incident on the third
and fourth quadrants located on the other side. Since the phase
difference occurs with respect to the tangential direction of the
information track in the far field image, it will be hereinafter
called "tangential phase difference". When an information is
recorded in a recording medium as irregularities of a surface with
a phase depth of an integral number times a quarter wave length,
the tangential phase difference becomes zero when focussing. But a
defocussing causes a tangential phase difference corresponding to a
defocussed quantity. On the other hand, when the pit phase depth
deviates from an integral number times a quarter wave length, the
tangential phase difference does not become zero even when
accurately focussing. This causes an offset in the tangential phase
difference. The offset in the tangential phase difference varies
depending on the deviation of the pit phase depth. When the far
field image is symmetrically formed with respect to the first and
second straight lines, the detection of the phase difference
between the sum of outputs from the first and third quadrants and
the sum of outputs from the second and fourth quadrants causes the
tangential phase difference to be canceled and not to become the
offset of the tracking error signal.
However, if the far field image formed on the photodetector is
asymmetrically displaced with respect to the first straight line,
the tangential phase difference is not canceled to become the
offset of the tracking error signal. Especially, when the pit phase
depth is, less than a quarter wave length, the offset of the
tracking error signal varies in a direction to increase the
tracking error. A bias of an objective lens in the radial direction
causes the offset of the tracking error signal develop to cause a
positive feedback so as to further expand the bias. Thus, if the
tracking control is released, the tracking control system runs away
and the carriage transferring the optical head also runs away.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical
information reproducing apparatus in which the offset of tracking
error signal varies little even if the image on a photodetector is
displaced when reading an information from a recording medium where
the information is recorded as succession of pits which have a
phase depth different from an integral number times a quarter wave
length.
In order to obtain the above object, the optical information
reproducing apparatus by the present invention is comprised as
follows. The far field image of a light spot by a detecting light
from a recording medium is divided into four quadrants by a first
straight line substantially parallel to a direction in which an
information track is extended and a second straight line
substantially perpendicular to the direction so that four
photocells can receive lights incident on the four quadrants
respectively. A tracking error is detected according to at least
one of two phase differences: a phase difference between outputs
from the photocells at the first and second quadrants located on
one side divided by the second straight line; and a phase
difference between outputs from the photocells at the third and
fourth quadrants located on the other side divided by said second
straight line. Alternatively, a tracking error is detected
according to a phase difference between a sum of a signal, which is
outputted from the photocell at the first quadrant photocell and
delayed for a specified time and a signal which is outputted from
the photocell at the third quadrant photocell and a sum of a
signal, which is outputted from the photocell at the second
quadrant photocell and delayed for a specified time and a signal
which is outputted from the photocell at the fourth quadrant
photocell.
Any deviation in the offset of tracking control signal gives no
adverse effect on tracking control even if the far field image of
the spot light on the photodetector is displaced. This is because
the tangential phase difference causes no influence.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a block diagram showing a preferred embodiment of this
invention.
FIGS. 2, 3 and 4 are block diagrams showing major parts of other
preferred embodiments of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1, an irradiation beam 21 radiated from a semiconductor
laser 1 and passed through a beam splitter 2 is converged on a
recording medium 20 by an objective lens 3 to form a light spot.
The reflected light from the recording medium 20 is collected by
the objective lens 3 to be a detecting beam 22. The detecting beam
22 is reflected by the beam splitter 2 and separated from the
irradiation beam 21. The detecting beam 22 is provided with
astigmatism by a cylindrical lens 4, and then forms a far field
image of the light spot on a photodetector 5. The photodetector 5
is divided into four quadrants by two straight lines parallel to Z
and X directions in the figure, and receives respective parts of
the far field image incident on the four quadrants by photocells
5a, 5b, 5c and 5d respectively to output signals corresponding to
the received light quantity.
Assume that in an information track, an information is recorded as
succession of pits with a phase depth less than a quarter wave
length, and on the photodetector 5 a direction in which the
information track extendedly exists is imaged in the direction of Z
in the figure. In this case, when a correctly focussed light spot
scans the information track, a phase difference is produced between
the output signals of the first quadrant photocell 5a and the
second quadrant photocell 5b, and those of the third quadrant
photocell 5c and the fourth quadrant photocell 5d. This means that
an offset of the tangential phase difference is produced. Here,
assume that the output signals from photocells 5a and 5b are in
advance in phase of those from photocells 5c and 5d.
Delaying circuits 6 and 7 delay the output signals from photocells
5a and 5b so as to cancel the offset of the tangential phase
difference. An adder 8 adds an output signal current from the delay
circuit 6 and that from the photocell 5c, and converts the
resultant current to a voltage. An adder 9 adds an output signal
current from the delaying circuit 7 and that from the photocell 5d,
and converts the resultant current to a voltage. A phase comparator
12 can detect tracking error by comparing phases of the output
signals of the adder 8 and the adder 9.
Before this phase comparison, high frequency componets are
extracted by means of high-pass filters 70 and 71 for removing low
frequency noises from the recording medium, and shaped in waveform
by means of waveform shapers 10 and 11 for making the phase
comparison easy.
In such a manner, the offset of the tangential phase difference is
removed by the delaying circuits 6 and 7, thus providing a correct
detection of tracking error even if the far field image is
displaced on the photodetector 5. A low-pass filter l3 removes
ripple component from the output signal of the phase comparator 12
to obtain a tracking error signal. A tracking control circuit 14
drives an actuator 18 in response to the tracking error signal to
control the position of the objective lens 3. Tracking control is
performed in this manner.
On the other hand, the detecting beam 22 is provided with
astigmatism, allowing a focussing error to be detected by a
difference between the quantity of light incident on the photocells
5a and 5c and that incident on the photocells 5b and 5d. Such a
focussing error detecting method is known as disclosed in U.S. Pat.
No. 4,023,033.
A differential amplifier 15 outputs a signal corresponding to a
difference between the output signals of the adder 8 and the adder
9. A low-pass filter 16 removes ripple component from this output
signal to obtain a focussing error signal. A focussing control
circuit 17 drives the actuator 18 in response to the focussing
error signal to control the position of the objective lens 3.
Focussing control is performed in this manner. With such a manner,
both of the focussing error and tracking error can be detected from
the same two signals which are output from the photodetector 5: the
former from the level difference, and the latter from the phase
difference. Accordingly, the photodetector 5 can be composed of
only four photocells and the adders 8 and 9 also be used in common
for the two error detections.
On the other hand, a preamplifier 19 amplifies a sum signal of all
of the photocells from an injection current into the substrate of
the photodetector 5 to obtain an information signal. In this
manner, even if the delaying circuits 6 and 7 are each composed of
a low-pass filter having capacitor and resistor as shown in the
figure, the frequency characteristic of the information signal will
not be deteriorated.
FIG. 2 shows another embodiment of the invention provided with
improved delaying circuits. In the embodiment of FIG. 1, the
delaying circuits 6 and 7 are adapted such that the output signals
of the photocells 5a and 5b are delayed for a specified time.
However, the range of the phase depth of the pits on the recording
medium is limited because a variation in the tangential phase
difference causes a variation in the optimum value of the delaying
time. In the embodiment of FIG. 2, the delaying time of the
delaying circuits is adaptively controlled so as to obtain the
effect of this invention even if the phase depth of the pits
varies.
Variable delaying circuits 30 and 31 are adapted such that the
delaying time is changed by means of a delaying time controller 44.
The delaying time controller 44 controls the delaying time as
follows. Buffer amplifiers 32 and 33 convert the respective signal
currents from the variable delaying circuits 30 and 31 into
voltages. Buffer amplifiers 34 and 35 directly convert the
respective signal currents from the photocells 5c and 5d into
voltages. Output signals from the buffer amplifiers 32 and 33 are
added to each other by an adder 38, and output signals from the
buffer amplifiers 34 and 35 are added to each other by an adder 39.
When the tangential phase difference is not optimally canceled by
the variable delaying circuits 30 and 31, a phase difference
remains in output signals from the adders 38 and 39. Thus, this
phase difference of the output signals from the adders 38 and 39
detected by means of a phase comparator 42 corresponds to an error
in the delaying time of the variable delaying circuits 30 and
31.
High pass filters 72 and 73, and waveform shapers 40 and 41
function similarly to the high pass filters 70 and 71, and the
waveform shapers 10 and 11 in FIG. 1. An integrator 43 integrates a
signal corresponding to the error in the delaying time from the
phase comparator 42. When an integrated value at the integrator 43
reaches a specified value, the delaying time controller 44 changes
the delaying time of the variable delaying circuits 30 and 31. In
this manner, the variable delaying circuits 30 and 31 are
controlled so as to precisely cancel the tangential phase
difference.
An adder 36 adds the signal which is outputted from the photocell
5a and delayed by the delaying circuit 30 to the output signal from
the photocell 5c. An adder 37 adds the signal which is outputted
from the photocell 5b and delayed by the delaying circuit 31 to the
ouput signal from the photocell 5d. Tracking error is detected from
a phase difference between output signals from the adders 36 and 37
similarly to the case in FIG. 1
In this embodiment, in order to provide a better matching of the
delaying time of the variable delaying circuits 30 and 31, the
delaying time of the circuits 30 and 31 is designed to be changed
discretely. If a sufficient matching of the delaying time is
provided, the delaying circuits may be adapted such that the
delaying time can be continuously changed.
Although the tangential phase difference is precisely canceled in
the above embodiment, it is not always necessary to precisely
cancel the tangential phase difference. The tangential phase
difference may be reduced to such an extent that the unstability of
tracking control is eliminated. When, in particular, the pit phase
depth is less than a quarter wave length, the offset of the
tracking error signal resulting from the movement of the far field
image on the photodetector 5 according to the movement of the
objective lens for tracking often causes the tracking control to be
in unstable condition as previously described. Accordingly the
delaying time of the variable delaying circuits 30 and 31 may be
adjusted so as to excessively cancel the offset of the tangential
phase difference. In this manner, the offset of the tracking error
signal developing when the objective lens 3 is biased for tracking
control is placed in a direction of negative feedback so as to
cancel the bias of the objective lens 3. Thus, even if the tracking
control is released, the tracking control system and the carriage
will not run away.
FIG. 3 shows a block diagram of a major part of still another
embodiment of the invention. In the embodiments of FIGS. 1 and 2, a
delaying circuit is provided to cancel the tangential phase
difference, while this FIG. 3 embodiment removes the influence of
the tangential phase difference by a different manner. That is,
this embodiment detects a tracking error by only the phase
difference between the quantity change of light incident on the
first quadrant and that of light incident on the second
quadrant.
Considering the fact that the far field image is formed almost
symmetrically with respect to the second straight line dividing the
photodetector 5, it is evident that, when a phase difference
corresponding to a tracking error is produced between the quantity
change of light incident on the first and third quadrants and that
of light incident on the second and fourth quadrants, the same
phase difference appears between the quantity change of light
incident on the first quadrant and that of light incident on the
second quadrant. Detection of the tracking error by the phase
difference between the quantity change of light incident on the
first quadrant and that of light incident on the second quadrant
totally eliminates the influence of the tangential phase
difference, allowing the offset of the tracking error detection to
be removed.
Specific operation of the embodiment of FIG. 3 is as follows.
Low-pass filters 60 and 61 remove such high frequency components as
to pass through the high-pass filters 70 and 71 from the output
signals of the third quadrant photocell 5c and the fourth quadrant
photocell 5d respectively. The adder 8 adds an output signal
current from the low-pass filter 60 and an output signal current
from the first quadrant photocell 5a, and converts the sum signal
current into a voltage. The adder 9 adds an output signal current
from the low-pass filter 61 and an output signal current from the
second quadrant photocell 5b, and converts the sum signal current
into a voltage. Output signals from the adders 8 and 9 are passed
through the high-pass filters 70 and 71 so that low frequency
components are removed, and then are shaped in waveforms by the
waveform shapers 10 and 11.
In this manner, the phase difference between output signals of the
waveform shapers 10 and 11 substantially becomes the same as the
phase difference between the quantity change of light incident on
the first quadrant and that of light incident on the second
quadrant. This is because the signal components corresponding to
the quantities of lights incident on the third and fourth quadrants
are almost removed by the low-pass filters 60 and 61, and by the
highpass filters 70 and 71. Thus, the phase difference
corresponding to the tracking error can be detected from the output
signals of the waveform shapers 10 and 11 by the phase comparator
12.
Since such a detection causes the tracking error to be immune to
the tangential phase difference, even if the objective lens moves
for tracking control the variation in the offset of the tracking
error becomes small as previously described.
The output signal of the adder 8 contains a sum of low frequency
components of the output signals from the photocells 5a and 5c, and
the output signal of the adder 9 contains a sum of low frequency
components of the output signals from the photocells 5b and 5d.
Thus, focussing error can be detected by the level comparator 15
from the output signals of the adders 8 and 9 similarly to the case
in FIG. 1.
FIG. 4 shows a block diagram of a major part of yet another
embodiment of the invention. In FIG. 4, buffer amplifiers 32, 33,
34 and 35 convert the output signal currents from the photocells
5a, 5b, 5c and 5d into voltages. Output signals from the buffer
amplifiers 32 and 33 are passed through high pass filters 74 and 75
so that low frequency components are removed, shaped in waveform by
waveform shapers 50 and 51, and then compared of their phases by a
phase comparator 54. In this manner, tracking error is detected
similarly to the case in FIG. 3.
On the other hand, tracking error is similarly detected also by
buffer amplifiers 34 and 35, high-pass filters 76 and 77, and a
phase comparator 55. By adding the output signals, varying
according to a tracking error, from the phase comparators 54 and
55, a more stable and less noisy tracking error signal than the
case in FIG. 3 can be obtained. Further, since the tracking error
is independently detected by the phase difference between the
quantity changes of lights incident on the first and second
quadrants and by the phase difference between the quantity changes
of lights incident on the third and fourth quadrants, the influence
of the tangential phase difference can be eliminated. Therefore,
the variation in the offset of tracking error detection can be made
small even if the objective lens is displaced for tracking
control.
As previously described, the present invention specifically
provides an apparatus to reduce the variation in the offset of
tracking error developing when the objective lens is displaced due
to eccentricity and the like of a recording medium for tracking
error detection by the phase difference tracking method, on the
basis of a discovery that the variation results from the tangential
phase difference. Any modification falls within the spirit and
scope of this invention insofar as such a modification is based on
the concept explained in connection with the above embodiments.
* * * * *